Heretofore, in an electrostatic recording process performed in, for example, a copy machine or printer, printing has been carried out by the steps of uniformly electrifying the surface of a photoconductor (latent image holder), forming an electrostatic latent image by projecting an image from an optical system onto the photoconductor to diselectrify the area exposed to light, then supplying a toner to the electrostatic latent image to form a toner image through electrostatic adhesion of the toner, and transferring the toner image onto a recording medium such as paper, OHP, or photographic paper.
Also in a color printer or color copy machine, printing is basically carried out in accordance with the above-described process, but in the case of color printing in which four color toners, magenta, yellow, cyan, and black, are used for reproducing color tones, a step of overlapping the color toners at a predetermined ratio is required to obtain an intended tone. In order to carry out the step, various methods have been proposed.
First, there is an image-on-image development system in which an electrostatic latent image is visualized with toners supplied onto a photoconductor in the same manner as monochrome printing; four color toners, or the magenta, yellow, cyan and black toners are overlaid one after another to develop the electrostatic latent image into a color toner image on the photoconductor. This system enables a relatively compact apparatus structure. However, gradation control is very difficult in this system, so there is a problem that high image quality cannot be obtained.
Second, there is a tandem system in which a color image is reproduced by developing latent images on four photoconductive drums with magenta, yellow, cyan, and black toners into magenta, yellow, cyan, and black toner images, and sequentially transferring the toner images from the photoconductive drums, which have been disposed in series, onto a recording medium such as paper. This system provides a good image, but the apparatus is large and expensive because the four photoconductive drums each having an electrification device and a development device are disposed in series.
FIG. 2 shows an example of the structure of a printing unit in an image forming apparatus for the tandem system. Four printing units, each printing unit containing a photoconductive drum 1, an electrification roll 2, a development roll 3, a development blade 4, a toner supply roll 5, and a cleaning blade 6, are disposed in series to correspond to yellow toner Y, magenta toner M, cyan toner C, and black toner B. The printing units are circularly driven by a driving roller (driving member) 9 to sequentially transfer the toners onto the sheet transported by a transfer/transport belt 10 thereby forming a color image. The transfer/transport belt is electrified and diselectrified by an electrification roll 7 and a diselectrification roll 8, respectively. In order to electrify a sheet to make it attached to the belt, an attraction roller (not shown) is used. These processes reduce the generation of ozone. The attraction roller transfers the sheet from a transport path onto the transfer/transport belt 10, and also fixes it thereon by electrostatic attraction. The transferred sheet can be separated from the belt only by curvature separation by lowering the transfer voltage to weaken the attraction power between the sheet and the transfer/transport belt.
The transfer/transport belt 10 may be made of a resistive material or a dielectric material, both of them having advantages and disadvantages. Since a resistive belt retains charges for only a short time, when the belt is used for transfer in an tandem system apparatus, charge injection caused by the transfer is low, and the increase in voltage is relatively small even with continuous transfer of four colors. In addition, even when repeatedly used for transfer of the succeeding sheet, the resistive belt does not require to be electrically reset because charges thereon have already been released. However, the resistance value of the resistive belt varies according to environmental variations, which affects the transfer efficiency, and the belt is susceptible to the thickness and width of the sheet.
On the other hand, the dielectric material belt does not naturally release injected charges, so that both of injection and release of charges have to be electrically controlled. However, since the belt stably holds charges, it securely attracts a sheet thereto and transports it with high accuracy. In addition, the dielectric constant is less dependent on the temperature and humidity, so that the transfer process is relatively stable against environmental variations. A disadvantage is that charges are accumulated in the belt by every transfer, which increases the transfer voltage.
Third, there is a transfer drum system in which a recording medium such as paper is wound around a transfer drum and rotated for four turns, in the rotations, magenta, yellow, cyan, and black toners on a photoconductor are sequentially transferred onto the recording medium to reproduce a color image. This system provides relatively high image quality. However, there is a problem that the type of the recording medium is limited, because it is hard to wind a heavy recording medium such as a postcard around the transfer drum.
As an alternative system to the above-described image-on-image development system, tandem system, and transfer drum system, an intermediate transfer system has been proposed. The intermediate transfer system provides favorable image quality with no increase in the size of the apparatus or limitation on the type of the recording medium.
More specifically, in accordance with the intermediate transfer method, an intermediate transfer member composed of a belt and a drum for transferring and temporarily holding a toner image is provided, and four photoconductors having a magenta toner image, a yellow toner image, a cyan toner image, and a black toner image formed thereon are disposed around the intermediate transfer member. The four color toner images are sequentially transferred onto the intermediate transfer member to form a color image on the intermediate transfer member, and the color image is then transferred onto a recording medium such as paper. The gradation is thus adjusted by superimposing the four toner images, so that high image quality is provided. In addition, the size of the apparatus is not significantly increased because the photoconductors are not required to be disposed in series as in the tandem system, and the type of the recording medium is not limited because the recording medium is not required to be wound around the drum.
FIG. 3 shows an example of the image forming apparatus for forming a color image using the intermediate transfer system, the apparatus including an intermediate transfer member in the form of an endless belt.
In FIG. 3, reference numeral 11 indicates a drum-shaped photoconductor which is allowed to rotate in the direction shown by an arrow in the figure. The photoconductor 11 is electrified by a primary electrifier 12, subsequently the portion exposed to an image exposure 13 is diselectrified, an electrostatic latent image corresponding to a first color component is formed on the photoconductor 11, the electrostatic latent image is developed by a developing device 41 with the first color magenta toner M, and the first color magenta toner image is formed on the photoconductor 11. Subsequently, the toner image is circularly driven by a driving roller (driving member) 30 to be transferred onto an intermediate transfer member 20 which is circularly rotating in contact with the photoconductor 11. In this case, the transfer from the photoconductor 11 onto the intermediate transfer member 20 is carried out by a primary transfer bias applied from a power supply 61 to the intermediate transfer member 20 at a nip portion between the photoconductor 11 and the intermediate transfer member 20. After the first color magenta toner image is transferred onto the intermediate transfer member 20, the surface of the photoconductor 11 is cleaned by a cleaning device 14, and thus the photoconductor 11 completes the first rotation to carry out the development and transfer operation.
Subsequently, the photoconductor 11 rotates for three turns, and, in the turns, a second color cyan toner image, a third color yellow toner image, and a fourth color black toner image are sequentially formed in that order on the photoconductor 11 by the developing devices 42 to 44, respectively, so that the four color images are superimposed on the intermediate transfer member 20 after the four turns, and thus a composite color toner image corresponding to the intended color image is formed on the intermediate transfer member 20. In the apparatus shown in FIG. 3, the positions of the developing devices 41 to 44 are sequentially changed after every turn of the photoconductor 11 to carry out development with the magenta toner M, cyan toner C, yellow toner Y, and black toner B in that order.
Next, a transfer roller 25 comes in to contact with the intermediate transfer member 20 having the composite color toner image thereon, and a recording medium 26 such as paper is fed from a paper feeding cassette 19 to a nip portion between the roller 25 and the member 20. Simultaneously, a secondary transfer bias is applied from a power supply 29 to the transfer roller 25, the composite color toner image is transferred from the intermediate transfer member 20 onto the recording medium 26, and is fixed thereon by heating to make a final image. The intermediate transfer member 20, after transferring the composite color toner image to the recording medium 26, is cleaned by a cleaning device 35 to remove transfer residual toner from the surface thereof, and is returned to the initial state to prepare for the next image formation.
In addition, there is a tandem intermediate transfer system which combines the tandem system and the intermediate transfer system. FIG. 4 illustrates an image forming apparatus of the tandem intermediate transfer system for forming a color image using an endless belt-shaped tandem intermediate transfer member.
In the apparatus shown in FIG. 4, a first development portion 54a to a fourth development portion 54d for developing electrostatic latent images on photoconductive drums 52a to 52d with yellow, magenta, cyan, and black toners, respectively, are sequentially disposed along a tandem intermediate transfer member 50. The tandem type intermediate transfer member 50 is circularly driven in the direction indicated by an arrow shown in FIG. 4, so that four color toner images formed on the photoconductive drums 52a to 52d of the development portions 54a to 54d are sequentially transferred onto the tandem intermediate transfer member 50 to form a color toner image thereon. The toner image is then transferred onto a recording medium 53 such as paper to be printed out.
In FIG. 4, reference numeral 55 indicates a drive roller or a tension roller for circularly driving the tandem type intermediate transfer member 50, reference numeral 56 indicates a recording medium feeding roller, reference numeral 57 indicates a recording medium feeding device, and reference numeral 58 indicates a fixing device for fixing an image on the recording medium by heating or other methods. Reference numeral 59 indicates a power supply device (voltage application unit) for applying a voltage to the tandem intermediate transfer member 50, and the power supply device 59 is adapted to reverse the applied voltage from positive to negative, and vice versa, depending on whether the toner images are transferred from the photoconductive drums 52a to 52d to the tandem intermediate transfer member 50, or from the tandem intermediate transfer member 50 to the recording medium 53.
In the above-described various image forming apparatuses, the conductive endless belt used as, for example, the transfer/transport belt 10, intermediate transfer member 20, or tandem type intermediate transfer member 50 is commonly a semiconductive resin film belt or a fiber reinforced rubber belt. Examples of the semiconductive resin film belt include a conductive endless belt described in Japanese Patent Application Laid-Open Publication No. 2002-132053 (Claims and other sections), which is composed primarily of a thermoplastic polyalkylene naphthalate resin, or a polymer alloy or polymer blend of the resin with another thermoplastic resin.
In addition, a semiconductive belt having a laminated structure for an electrophotographic apparatus is described in Japanese Patent Application Laid-Open Publication No. 2004-157289 (Claims and other sections), which is composed of a thin cylindrical elastic belt base having a surface layer on the external surface thereof, the surface layer being composed primarily of a copolymer of a fluorine-modified acrylate and another acrylate, a fluorinated olefin-based polymer, and a copolymer of methyl methacrylate and another monomer containing a (meth)acryloyl group. Each of the tensile breaking strength, tensile breaking elongation, and modulus of elasticity of the semiconductive belt has a specified value.